1. Field of the Invention
The present invention relates to an optical pulse signal generator system that provides an optical pulse signal coded in the differential phase shift keying (DPSK) scheme with an optical time division multiplexing (OTDM), thus generating and outputting an optical time division multiplexing differential phase shift keying (OTDM-DPSK) signal, and more particularly to a system for detecting an optical carrier phase difference between optical pulses in an OTDM-DPSK signal and to a system for controlling an optical carrier phase difference between optical pulses.
2. Description of the Background Art
In the field of optical communications technology, the intensity modulation-direct detection (IM-DD) scheme and the balanced detection scheme are proposed, see, for example, R. Ludwig, et al., “160 Gbit/s DPSK-Transmission-Technologies and System Impact” Proc. 30th European Conference on Optical Communication (ECOC 2004), Tul. 1, 1.
In the IM-DD scheme, a receiver end uses a photo diode to determine the envelope strength of an optical carrier of a received signal to detect the received signal. One typical example is optical communications using an optical pulse signal coded in the amplitude shift keying (ASK) scheme. Although the intensity modulation is generally referred to as the amplitude shift keying (ASK) or the on and off keying (OOK), the following description simply refers to the intensity modulation as the ASK.
In the balanced detection scheme, a receiver end uses a balanced detector to sensing an electrical signal with amplitude twice as large as in the IM-DD scheme to detect the received signal. One typical example is optical communications using an optical pulse signal coded in the DPSK scheme.
The IM-DD scheme has advantages that it allows for transmitting and received signals using a simple system and it may apply to an intensity recovering system incorporating an optical amplifier. The advantages allow the scheme to be widely used. The DPSK scheme uses two variables of zero and π to modulate an optical pulse string to produce and output a transmission signal, and the receiver end divides a received signal into first and second received signals, providing the first received signal with a time delay corresponding to a period of time occupied by one optical pulse, i.e. one bit, on the time axis, adding the first received signal having the time delay and the second received signal to each other, and providing the resulting received signal with the balanced detection. Hereafter, n is a positive integer, and 1/n of the time occupied by one optical pulse on the time axis may be referred to as a time corresponding to 1/n bit. A transmission signal (optical pulse signal) resultant from modulation of an optical pulse string using two variables of zero and π may also be referred to as a transmission signal coded in DPSK scheme or an optical pulse signal coded in DPSK scheme.
An optical pulse and the phase of an optical pulse will be described in more detail below. An optical pulse is observed as a change in light strength or intensity. An optical pulse is represented as the envelope of the amplitude waveform of the electric field vector of an optical carrier. In the following discussion, therefore, the temporal waveform of an optical pulse is directed to a representation of the envelope of the amplitude waveform of the electric field vector of an optical pulse.
The phase of an optical carrier for an optical pulse is directed to a representation of the relative phase of the peak of an optical carrier with respect to the peak of the envelope of the electric field vector of the optical pulse, and is referred to as the optical carrier phase or the absolute phase. Strictly speaking, the optical carrier phase or the absolute phase may be referred to as the carrier-envelope phase (CEP). The phase of an optical carrier for an optical pulse will hereafter be referred to as an optical carrier phase. Each peak of the envelope of the electric field vector of an optical pulse includes a great many peaks of the optical carrier.
For example, a wavelength of 1.5 μm for the optical carrier corresponding to a frequency of approximately 2×1014 Hz. If the optical pulse has its repetition frequency approximately equal to 40 GHz, the frequency is 4×1010 Hz. Therefore, each peak of the envelope of the electric field vector of the optical pulse includes 5000 [=(2×1014)/(4×1010)=5×103] peaks of the optical carrier.
The modulation of the optical pulse using the two variables of zero and π corresponds to maintaining the phase of the electric field vector of an optical carrier (the phase for an optical carrier) in an optical pulse or shifting the phase by π with respect to the envelope of an optical carrier. In other words, the modulation of an optical pulse using the two variables of zero and π corresponds to maintaining the optical carrier phase or shifting the phase by π.
It is known that the balanced detection scheme applied to the DPSK scheme may improve the receiver sensitivity by 3 dB or more compared to the intensity modulation-direct detection scheme applied to the ASK scheme, see, for example, R. Ludwig, et al., indicated above.
R. Ludwig, et al., discloses an OTDM communications system in DPSK scheme. The system operates as follows. The system generates a DPSK signal of 40 Gbit/s. The system divides the DPSK signal into four signals. The system provides, except the first line signal (corresponding to one channel), the second, third and fourth line signals (corresponding to three channels) with a time delay corresponding to ¼, 2/4, ¾ bit, respectively. The system multiplexes the first to fourth line signals (including the first line signal without time delay), thus producing the OTDM signal. Among the four line signals, therefore, the first line is of the actual signal and the remaining second, third and fourth lines may be considered as copy signals of the first line signal. The practical generator system requires the first to fourth line signals to be different. The OTDM communications system in DPSK scheme disclosed in R. Ludwig, et al., may thus not be directly used to provide a system that generates practical OTDM-DPSK signals.
With reference to FIG. 1, a description will be given of an exemplified system in which the practical multiple-channel optical pulse signals coded in DPSK scheme are provided with the OTDM to generate an optical time division multiplexing differential phase shift keying signal, the system being sometimes referred to as “OTDM-DPSK signal generator system”. The system is configured according to the OTDM communications system in DPSK scheme disclosed in R. Ludwig, et al. FIG. 1 is a schematic block diagram of the configuration of an OTDM-DPSK signal generator system 10.
The OTDM-DPSK signal generator system 10 includes an optical splitter 12, a first phase modulator 14, a second phase modulator 16, a ½-bit delay device 18, and an optical coupler 20. The OTDM-DPSK signal generator system 10 is supplied with a transmission signal from the first and second modulator drivers 22 and 24.
An optical pulse string 11 including optical pulses at regular intervals on the time axis enters the OTDM-DPSK signal generator system 10. The optical pulse string 11 is split by the optical splitter 12 into a first and a second optical pulse string 13-1 and 13-2. The first and second optical pulse strings 13-1 and 13-2 enter the first and second phase modulators 14 and 16, respectively.
In the first and second phase modulators 14 and 16, the first and second optical pulse strings 13-1 and 13-2 are decoded in DPSK scheme using the transmission signals 23 and 25 supplied from the first and second modulator drivers 22 and 24, thus generating and outputting the first and second phase shift keying signals 15 and 17, respectively.
The second phase shift keying signal 17 enters the ½-bit delay device 18 that provides the signal 17 with a time delay corresponding to ½ bit, thus generating and outputting a delayed second phase shift keying signal 19. The first phase shift keying signal 15 and the delayed second phase shift keying signal 19 are multiplexed by the optical multiplexer 20, thus generating and outputting a multiple phase shift keying signal 21. The OTDM-DPSK signal generator system 10 thus has a function of being responsive to the optical pulse string 11 to convert the two-channel transmission signals supplied from the first and second modulator drivers 22 and 24 into the DPSK signal, and providing the two-channel DPSK signals with the OTDM to output a resultant signal.
Although FIG. 1 shows an example where the two-channel OTDM is provided, the OTDM with any number of channels may be applied. For example, the four-channel OTDM may be provided by providing the first to fourth channel optical pulse signals coded in DPSK scheme with respective time delays corresponding to zero, ¼, 2/4, and ¾ bit, and multiplexing resultant signals.
In order that the OTDM-DPSK signal generator system 10 may operate as a system that generates an OTDM-DPSK signal, it is required that the first and second optical pulse strings 13-1 and 13-2 generated in the optical splitter 12 experience no phase modulation other than the phase modulation provided by the first phase modulator 14, the second phase modulator 16, and the ½-bit delay device 18. In the optical multiplexer 20, therefore, the optical carrier phase differences between the optical pulses in the first phase shift keying signal 15 and between the optical pulses in the delayed second phase shift keying signal 19 must not take a value other than zero or π.
Temperature variations or the like cause fluctuations in the optical path length, however, of an transmission line over which the first and second optical pulse strings 13-1 and 13-2 propagate, and the transmission line over which the first and second phase shift keying signals 15 and 17 propagate, and the transmission line over which the delayed second phase shift keying signal 19 propagates. It is extremely difficult for the conventional technologies to reduce the fluctuations in the optical path length below a level that is negligible compared to zero or π in terms of the optical carrier phase. Therefore, a need exists for a method of detecting and controlling an optical carrier phase difference between optical pulses in an optical pulse signal.
One example of the method of detecting and controlling an optical carrier phase difference is disclosed by Japanese patent laid-open publication No. 2005-006175. The '175 Japanese publication discloses a method that splits a portion from an optical pulse signal coded in ASK scheme, leads the split optical pulse signal to an interferometer, and observes the intensity of the interference light output from the interferometer.
The method disclosed in the '175 publication will be described below with respect to, for example, the optical pulse signal in carrier-suppressed-return-to-zero (CS-RZ) format. An optical pulse signal in CS-RZ format is generated from an optical pulse string (which may be referred to hereafter as “the CS optical pulse string”) coded in ASK scheme, the optical pulse string including adjacent optical pulses with an optical carrier phase difference of π between them.
In order to divide a portion of the optical pulse signal in CS-RZ format according to its intensity to detect an optical carrier phase difference, the signal is led to an optical carrier phase difference detection system. The optical carrier phase difference detection system divides the input optical pulse signal in CS-RZ format into two sub-signals. The system then provides one sub-signal with one bit delay, and then causes the two sub-signals to interfere again. When the interference signal is thus output from the optical carrier phase difference detection system, the ideal optical pulse signal in CS-RZ format will cause output light of zero intensity, the ideal optical pulse signal including adjacent optical pulses with the optical carrier phase difference between them taking no other value than π.
This is for the following reasons. The CS-RZ format includes smallest optical pulses that invert their phases between zero and π on a bit-by-bit basis. When the optical pulse string is divided into two sub-strings, one sub-string is provided with one bit delay and the two sub-strings are then caused to interfere, the optical pulses both of which have an optical carrier phase of π will interfere.
When, in an optical pulse signal in CS-RZ format, the adjacent optical pulses have between them an optical carrier phase difference that takes a value other than zero and π, the optical carrier phase difference detection system outputs an interference signal having non-zero intensity. Specifically, when the optical pulses have between them an optical carrier phase difference that is offset from zero or π by an angle of φ (zero<φ=<π), the optical carrier phase difference detection system outputs an interference signal having an intensity that increases as φ approaches zero and that has the maximum intensity for φ=zero. Thus, the time average of the intensity of the interference signal output from the optical carrier phase difference detection system may be monitored to know the value of φ. A feedback control may be performed so that the time average of the intensity of the interference signal output from the optical carrier phase difference detection system is always of the minimum. This may generate the ideal optical pulse signal in CS-RZ format in which the optical carrier phase difference between the adjacent optical pulses takes no other value than π. Hereafter, φ may be referred to as the size of phase fluctuations.
The optical carrier phase difference detection method that uses the optical pulse signal in CS-RZ format may not apply, however, to the OTDM-DPSK signal output from the OTDM-DPSK signal generator system described with reference to FIG. 1. The reason will be described below with reference to FIG. 2, parts (A), (B), and (C). FIG. 2, parts (A), (B), and (C) show the temporal waveforms of an optical signal at each location in the OTDM-DPSK signal generator system. Part (A) shows the temporal waveform of the first phase shift keying signal 15. Part (B) shows the temporal waveform of the delayed second phase shift keying signal 19. Part (C) shows the temporal waveform of the multiple phase shift keying signal 21. The temporal waveforms shown in parts (A), (B), and (C) include the envelopes of the amplitude waveforms of the electric field vector of the optical carrier, but only the positive halves of the envelopes are depicted with the negative halves thereof omitted from illustration. The x-axis is the time axis and the y-axis represents the amplitude, both on an arbitrary scale.
The first phase shift keying signal 15 shown in FIG. 2, part (A) and the delayed second phase shift keying signal 19 shown in part (B) interfere with each other when the optical pulses in the signals 15 and 19 have the optical carrier phases of zero and φ, φ and π, zero and (π+φ), and π and (π+φ), respectively. When the ideal optical pulse signal in CS-RZ format is provided with the OTDM to be generated in the vorm of multiple phase shift keying signal 21, the phase fluctuations have a size of zero, i.e. φ=zero. However, because fluctuations in optical path length occur in each optical transmission line over which the optical signal of the OTDM-DPSK signal generator system propagates, some control is required to keep φ=zero.
The first and second phase modulators 14 and 16 modulate the optical pulses in the CS optical pulse string by zero or π as the optical carrier phase. Specifically, the modulators 14 and 16 provide the optical pulses in the CS optical pulse string without shift of the optical carrier phase of the optical pulses (zero modulation), or with π shift of the optical carrier phase of the optical pulses (π modulation).
The optical carrier phase of the optical pulses in the first and delayed second phase shift keying signals 15 and 19 are provided with, after the modulation in the first and second phase modulators 14 and 16, respectively, no phase change, shown as zero in FIG. 2, parts (A) and (B), or a phase change of π shift, shown as π in parts (A) and (B). In addition to the phase modulation, the optical carrier phase of the optical pulses of the first and delayed second phase shift keying signals 15 and 19 is added with the phase fluctuations φ caused by the fluctuations in the optical path length or the like.
Therefore, when the first phase shift keying signal 15 shown in FIG. 2, part (A) and the delayed second phase shift keying signal 19 shown in part (B) are added to or combined with each other, the resultant multiple phase shift keying signal 21 has a temporal waveform as shown in part (C). The reason will be described below.
In FIG. 2, parts (A), (B), and (C), the time axis is sectioned by vertical broken lines into time intervals each including one optical pulse. In parts (A), (B), and (C), the leftmost optical pulses will now be considered. Among the optical pulses in the first phase shift keying signal 15 shown in part (A), the leftmost optical pulse has an optical carrier phase of zero. Among the optical pulses in the delayed second phase shift keying signal 19 shown in part (B), the leftmost optical pulse has an optical carrier phase of φ. When the two pulses interfere, the resultant optical pulse has a non-zero finite amplitude, referred to as a first amplitude, as shown in part (C) as the leftmost one of the optical pulses in the multiple phase shift keying signal 21.
Now, the second left optical pulses will be considered. The optical pulse with an optical carrier phase of φ and the optical pulse with an optical carrier phase of π interfere, thus generating an optical pulse having amplitude, referred to as a second amplitude, different from the first amplitude.
Therefore, when viewing the amplitudes of the optical pulses in the signals shown in FIG. 2, parts (A), (B) and (C) from left to right, the following may be concluded. The optical pulse having the first amplitude is generated when the optical pulses in the first and delayed second phase shift keying signals 15 and 19 interfere with each other with their optical carrier phases of zero and φ, and π and (π+φ), respectively. The optical pulse having the second amplitude is generated when the optical pulses in the first and delayed second phase shift keying signals 15 and 19 interfere with each other their optical carrier phases of φ and π, and zero and (π+φ), respectively.
It can therefore be seen that the multiple phase shift keying signal 21 includes two types of optical pulses: the first size optical pulses and the second size optical pulses. The two types of optical pulses in the multiple phase shift keying signal 21 have amplitudes which are the function of φ that is caused by the fluctuations in the optical path length or the like. When φ approximates zero, the first amplitude is larger than the second amplitude. When φ approximates π, the second amplitude is larger than the first amplitude. FIG. 2, part (C), shows an example where the first amplitude is larger than the second amplitude, i.e. φ approximates zero.
Therefore, when φ changes between zero and π, the first and second amplitudes change in a relationship that when one amplitude decreases the other increases. The average intensity of the multiple phase shift keying signal 21 thus remains unchanged, thus preventing the optical carrier phase difference detection method that uses the optical pulse signal in CS-RZ format from detecting the phase difference φ.